For quick estimates ONLY
1 Litre of water (at normal pressure and temp) = 1 kilogram
so
1000 Mil = 1000 grams.

Of course this all changes when you bring into the effects of Density, Pressure and Temperature on the given material.

To go from volume to mass, you multiply the volume by the density.

,

where m is the mass, V is the volume, and is the density.

For example, pure liquid water has a density of 1 g/mL, so 5 mL of water has a mass of 5 g. Gold on the other hand, has a density 19.3 g/mL, so 5 mL of gold would have a mass of 96.5 g.